It is the intention of the proposed work to characterize both by biochemical and EM immunocytochemical procedures the protein composition of the vertebrate rod outer segment plasma membrane. The long-term objective of the work is to determine whether sorting of outer segment proteins occurs during disc morphogenesis, thereby generating a plasma membrane which differs compositionally from the discs. Sorting of gene products, as a general feature of cell structure and long-term maintenance, may be relevant to slow-onset retinal degenerations which more closely resemble the human condition, retinitis pigmentosa. The proposal also is concerned with the identification of plasma membrane constituents which participate in the light-sensitive permeability mechanism and renewal processes of the vertebrate rod outer segment, and any relation they may have to maladies during disc shedding and phagocytosis. Three basic strategies for identifying outer segment plasma membrane components are proposed. The first involves probing the extracellular surface of intact rod outer segments with antibodies and lectins already available. The second consists of preparing enriched plasma membrane fractions for characterization by gel electrophoresis and subsequent antibody production. The third strategy examines similarities between the extracellular surfaces of photoreceptors and other cells. All three strategies depend upon both biochemical and immunocytochemical evaluations for establishing the identity of plasma membrane constituents.